1. Field of the Invention
The present invention relates to electroluminescent structures and, more particularly, to an active matrix electroluminescent display having a plurality of high-performance pixels.
2. Description of the Related Art
Electroluminescent (EL) displays produce light when an alternating current (AC) voltage is applied across a phosphor film sandwiched between a pair of electrodes. If an organic material is used, a similar structure is employed however excitation of the organic material is performed in a different manner, for example using DC current. Referring to FIG. 1, electroluminescent light originates from metal activator atoms that are introduced into a phosphor film 12 and excited by energetic electrons as they move across the semi-conducting phosphor film 12. Since the phosphors employed have large band gaps, visible radiation produced (indicated by arrows) passes through film 12 without absorption and out of the stack through a transparent electrode 14.
The typical EL film stack contains two dielectric layers 16 and 18, one at each electrode interface, i.e. one for transparent electrode 14 and one for electrodes 20. These dielectric layers limit the current through the structure and prevent a catastrophic breakdown should a phosphor imperfection produce a conductive path through film 12. Dielectric layers 16 and 18 also store charge, increase the internal electric field and reduce the effective turn-on voltage of the phosphor. High dielectric constant films are often used to enhance the effect and increase the luminous efficiency of EL displays.
Compact high-resolution displays have been produced with on-chip scanning and pixel control circuitry. In these "active matrix" displays, the necessary dielectric, phosphor and transparent electrode layers are deposited and defined as a single rectangle over the entire pixel array. Referring again to FIG. 1, individual pixel electrodes 20 are controlled by switching a transistor 22 which blocks the AC phosphor excitation voltage 24 when "off" and allows passage current through the phosphor when "on". Pixel electrodes 20 are positioned directly over the controlling transistors, to maximize resolution.
Referring to FIG. 2, one example of a control circuit is schematically shown. The control circuit includes a low voltage access transistor 32 connected to a data line 34. The state of a pixel 40 is programmed by asserting a select line 36 to turn on access transistor 32. The voltage on data line 34 is then stored onto a hold capacitor 38. A logic signal turns on a blocking transistor 30 to thereby allow conduction current to illuminate the pixel. Likewise blocking transistor 30 may be turned off by a different logic signal to maintain the pixel in an "off" state. Pixel 40 includes two electrodes and an EL stack as described with reference to FIG. 1.
Referring again to FIG. 1, in a typical AMEL display, a plurality of pixel electrodes 20 are formed as "islands" in a layer insulated from a silicon-on-oxide (SOI) structure in which the active semiconductor circuits are formed. These semiconductor circuits are connected to the pixel electrodes through respective contact holes formed in the insulating layer. Electroluminescent (EL) stack 12 is formed over the pixel electrodes 20, and this EL stack is a "sandwich" of EL material, typically phosphor, between top 16 and bottom 18 insulated layers.
A transparent conductive layer forms a transparent electrode 14, which receives the high voltage source 24, is provided on top of the EL stack; and this entire thinfilm structure is on a base or substrate 28 formed of silicon, glass, quartz or other suitable material.
Referring again to FIG. 2, in operation, low voltage access transistor 32 controls high-voltage (blocking) transistor 30, and the high-voltage transistor 30 turns on and "addresses" its associated pixel to illuminate the adjacent (or proximate) electroluminescent material in response to a signal received via a common pixel electrode (transparent electrode 14 of FIG. 1), the signal being in the order of 100 to 400 volts (AC or DC).
Such an overall arrangement is disclosed in a U.S. Pat. No. 5,485,055 ('055) to T. Keyser and assigned to the assignee of the present invention; moreover, in the '055 patent, the array of pixel electrodes has a uniformly textured surface for an enhanced brightness of the display. The '055 patent is incorporated herein by reference.
AMEL display technology is also disclosed in the following publications:
R. Khormaei, et al., "11.3: High-Resolution Active-Matrix Electroluminescent Display," SID 94 DIGEST 137; R. Khormaei, et al., "42.3: A 1280.times.1024 Active-Matrix EL Display," SID 95 DIGEST 891; and L. Arbuthnot, et al., "24.3: A 2000-lpi Active-Matrix EL Display 374, " SID 96 DIGEST, which are all incorporated herein by reference.
In the prior art, active matrix electroluminescent displays depend on an internal pixel capacitor or hold capacitor 38 to maintain the high voltage blocking transistor 30 in the desired state between write cycles. For the grounded DMOS structure the source of blocking transistor 30 and an electrode of hold capacitor 38 are grounded. The conventional method for creating the pixel or hold capacitor 38 is to deposit a thin (2000 .ANG.) layer of oxide between a first metal layer and a second metal of the structure wherein each metal layer act as an electrode of hold capacitor 38. Although this method is reasonably effective for larger pixels, for example, 24 .mu.m pixels, as pixel size is reduced, the area available to develop the necessary capacitance to maintain the blocking transistor in the specified state becomes increasingly small. As a result, affected pixels tend to "turn on" when written "off" and thereby degrade image quality.
Therefore, a need exists for an active matrix electroluminescent display having a plurality of high-performance pixels wherein improved capacitance is achieved thereby permitting reduced pixel size.